The objective of the proposed research is to define more precisely the roles bone marrow plays in the immune response of rodents. By achieving partial or complete ablation of hemopoiesis in the bone marrow through local radiation damage caused by 89Sr incorporation into bone, the contribution by the total bone marrow organ to the production of various types of lymphocytes will be investigated. The classes of lymphocytes will be defined in terms of their rates of turnover, life span and cell surface receptors. The same system will be adopted for studying the traffic of various subpopulations of lymphocytes through the marrow. These experiments will make use of parabiotic union between 89Sr-treated and normal rodents in which subpopulations of lymphocytes have been specifically marked with 3H-TdR or 125IUdR. The cell surface markers of lymphocytes thus labeled will be demonstrated on radioautographs after the cells are allowed to form rosettes with bacteria or erythrocytes that have been coated with antisera specific for receptors on the lymphocyte surface. The studies will provide an estimate of the turnover rate of T and B lymphocytes of varying maturity in tissues of the lymphomyeloid system. Combination of 89Sr treatment with splenectomy or thymectomy will define the role of each of the organs in the maintenance of potentially immunocompetent cells in the periphery. The same experimental system will be used to define the origin and nature of immunocompetent cells that accumulate in the marrow following immunization.